New EFI with "Hanging" Idle Problem

I finally completed my MAF install a few days ago. After a couple of bonehead maneuvers on stabbing the distributor I turned the key and got full run after one crank. I let the engine run at idle for about 30 minutes and then took it out for a drive. Everything seems to be working perfectly except for the return to idle after I let off of the gas. Instead of just falling off instantly like you would expect it kind of slowly ramps down to low idle. Its pretty irritating when shifting gears.

I did a search and found a few people calling this a "hanging idle" but I couldn't find much in the way of advice. The guys at BC (who are great for tech stuff, thanks) told me to check the VSS wiring, TPS voltage and clean the IAC valve. I adjusted the TPS to .95v and cleaned the IAC with no change. The VSS is correct (and the engine does the same thing if you rev it up sitting still). Does anyone else have any other pointers?

Here are my specifics:

'76 351W
GT-40 lower
'96 Explorer upper
Ryan's harness
'93 Mustang TPS
'96 IAC from a Bronco 5.8
'93 Mustang MAF
A9M ECM

I haven't got the check engine light in yet so I haven't pulled codes, which I know I should before I ask for help. I'm just looking for some pointers before I hit the garage today. TIA

are you using a throtle cable and a spring

Possible the ICS?

I'm using the BC throttle cable and I added a second return spring. I read about some people having problems with the stock throttle body return spring. The engine would never idle down until I put the second spring on. I think the EB pedal assembly is just too heavy.

If by ICS you mean the idle air control valve, that is starting to look like the problem. Is there any guide to testing the output?

Two things:

1. Try disconnecting the idle air bypass valve and running it. The computer uses the IAB (or IAC, or ICS, or whatever) valve to control idle speed.

2. Another possibility- the A9M is an automatic transmission computer, like the A9P. They supposedly have code built into them to drop the idle more slowly than the manual computers to deal with the torque converter drag (I think). The first computer I used on mine was an A9P. I also have a manual transmission and I had a mild hanging idle problem that drove me nuts. Switching to an A9L (manual computer) cured it. If you have access to a manual tranny computer you might try swapping.

I have something similar where when I coast down to a stop the engine "hangs" anywhere from 1100-1500rpm for a about 10-15 seconds after rolling to a stop, I have to blip the throttle to bring it down to ~800. I'm running a manual tranny on an A9P. I was going to try and add a second throttle return spring to see if it is the throttle plate not fully closing or if it's the IAC and the Auto computer telling it to do this.

Did you get the spring from BC the first few they sold did not have one all of the new ones do and they posted that if anyone was missing one to give them a call. It fixed mine the spring on the used TB I had was weak, that is what I get for using used stuff

Thanks for all the help. I disconnected the IAC this afternoon and it wouldn't idle at all unless you had your foot in it. In this case my 11 year-old had his foot in it, good boy. Anyway, the IAC valve I had was shot and I replaced it. I covered the bypass holes in the throttle body with a plate and watched the valve itself during a run. The old valve didn"t budge but the new one in the box moves all over to try to set idle. I still have the idle drag and I still get a #81 code that indicates a bad solenoid. I will erase the CM codes tomorrow and try again. It shouldn"t matter but its worth a try.

Lars -
I knew the A9M was an automatic computer and I wondered about the idle issue. The code theory makes sense, if the torque converter is spinning it will take more energy to keep the engine running than if it is a manual that gets an instant disconnect when the clutch is engaged. I don't have a manual computer, but all it takes is money!

I did not receive a new spring with the BC cable. However, I put the return spring from my carb on the throttle body. I can guarantee that it is closing all the way. It wouldn't before I added the spring but it does now.

The bottom line is it still works better than the carb ever did. At least I get an instant cold start and idle (even if it's high) at 15 degrees ambient. It beats the crap out of the choke - pump - pump - pumppumpump - chug - pump-pump rough idle I used to get.

Again, thanks to all for the help.

Brad

i'm curious, did you ever resolve your idle issue? i am experiencing the same thing.

I just unhooked the IAC and used the screw to set the idle.
I have an auto computer and a manual trans.

wasent really a problem with the T-18, but with the NV3550 Im in the next gear and the engine wont drop below 1200.

I also didnt like the way the IAC revved the engine when you first crank it, mine would rev to 1500 or so and stay for about 20 seconds, on a cold engine I didnt like that, its probably some emissions crap, to heat up the cat quickly.

I like it fine without the IAC at all, Just have to feather it a bit when its first cranked.

thanks for the reply. i disconnected mine today and started it up cold it was fine. idle stills seems to hang just a little so i'll continue to look for leaks. do you have your egr hooked up? i'm using the A9L with a C4. i just love the way this engine just starts right up and runs so smooth. can't wait to drive it.

I do not have the EGR hooked up,

I also have a cyl with very low compression, which is probably messing up the 02 sensors, I may have to start running mine with the 02's unplugged to keep it in the open loop program.

The EFI will spoil ya for sure, I had a 66 Thunderbird and spend many fruitless attempts trying to teach the wife how to start and drive a vehicle with a carb, cause she's never driven one, ever.